1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image that is suitable for forming a high quality image by an electrophotographic process, a process for producing the same, and a process for forming an image.
2. Description of the Related Art
A process for visualizing image information through an electrostatic latent image is being widely applied to various fields. In the electrophotographic process, an electrostatic latent image is formed on a photoreceptor through a charging step and an exposing step, and the electrostatic latent image is visualized through a developing step, a transferring step and a fixing step.
In the electrophotographic process, an electrostatic latent image formed on a photoreceptor through a charging step and an exposing step is developed with a developer and then transferred. The toner on a fixing material through the transferring step is heated and melted by a fixing member having a heating unit in the fixing step, whereby the toner image is fixed on the surface of the fixing material. In the fixing step, the fixing member heats not only the toner but also the fixing material to the necessary temperature, so as to fix the toner on the fixing material. When the heating of the fixing material is insufficient, so-called cold offset occurs, in which only the toner is melted, and the toner is adhered on the fixing member. When the heating is excessive, so-called hot offset occurs, in which the viscosity of the toner is decreased, and a part or the whole of the fixed image is adhered on the fixing member. Therefore, the heating by the fixing member necessarily falls within a fixing region, in which the cold offset and the hot offset do not occur.
According to an increasing demand for energy saving, the fixing temperature of the toner is necessarily decreased in order to realize energy saving of the fixing step, which consumes a certain extent of the consumed electric power of a duplicator, and enhancement of the fixing region. The decrease in the fixing temperature of the toner not only realizes the energy saving and the enhancement of the fixing region, but also shortens the so-called warm-up time, i.e., the latency time until the surface of a fixing roll reaches the temperature capable of conducting fixing upon turning on a duplicator, and enhances the service life of the fixing roll.
When the fixing temperature of the toner is lowered, it brings about decrease of the glass transition point of the toner to cause such a problem that the storage stability of the toner is deteriorated, and thus both the properties cannot be attained at the same time. In order to realize both the low fixing temperature and the storage stability of the toner, it is necessary that the toner has the so-called sharp melt property, i.e., the viscosity of the toner is quickly lowered at a high temperature region while the glass transition point of the toner is maintained at a high temperature.
However, because a resin used in the toner generally has fluctuation ranges in the glass transition point and the molecular weight, it is necessary that the composition and the molecular weight of the resin are uniformed to obtain the sharp melt property. In order to obtain such a resin, the molecular weight of the resin is necessarily uniformed by employing a special production process or by treating the resin with chromatography, whereby the production cost of the resin is considerably increased. Furthermore, it is not preferred from the standpoint of environmental protection since an unnecessary portion of the resin is formed.
As a method for lowering the fixing temperature of the toner, the use of a crystalline resin as the binder resin is proposed (as described in Japanese Patent Laid-Open Nos. 129867/1987, 170971/1987, 170972/1987, 205365/1987, 276565/1987, 276566/1987, 38949/1988, 38950/1988, 38951/1988, 38952/1988, 38953/1988, 38954/1988, 38955/1988, 38956/1988, 1217/1993, 148936/1994, 194874/1994, 5056/1993 and 112715/1993).
Although the fixing temperature can be lowered by these methods, since the gradient of the viscosity of the resin with respect to the temperature change is large, the sufficient viscosity cannot be obtained upon production of the toner, for example, upon kneading, and thus the dispersibility of a colorant and a releasing agent in the resin is not stabilized, whereby such a problem occurs that a toner having unevenness in the coloring property and the fixing property is liable to occur. Furthermore, pulverization of the kneaded product becomes difficult to cause such a problem that a toner having a small particle diameter is difficult to be obtained.
In order solve the problems, a method can be employed in that an auxiliary agent, such as a thickening agent and a pulverizing aid, is added, but is not preferred since these auxiliary agents are dispersed in the resin to deteriorate the crystallinity of the binder resin.
In recent years, an aggregation and coalescence process is proposed as a process for producing a toner, the particle shape or the surface composition of which is controlled according to the purpose (Japanese Patent Laid-Open Nos. 282752/1988 and 250439/1994). The aggregation and coalescence process is conducted in the following manner. A resin particle dispersion is produced by an emulsion polymerization process or a dispersion emulsification process, and separately, a colorant dispersion having a colorant dispersed in a solvent is produced. The dispersions are mixed to form aggregated particles having a diameter corresponding to a toner particle diameter and then subjected to heating and fusing to obtain toner particles. According to the aggregation and coalescence process, the toner shape can be arbitrarily controlled from an irregular shape to a spherical shape by selecting the conditions for heating temperatures.
It is general in the aggregation process that the resin particles are heated to a temperature near the glass transition temperature to partially melt the surface of the resin particles, whereby the aggregated particles are easily produced, and the crystalline resin can be subjected to the formation of aggregated particles. However, because the surface of the crystalline resin particles suffers great change in viscosity particularly near the melting point, the temperature range, within which the surface of the resin particles can be partially melted, is narrow in comparison to an ordinary noncrystalline resin. Thus, in the case where the aggregation temperature is low, the aggregated particles are unstable and easily broken, and in the case where the aggregation temperature is high, the particles are easily grown to cause a problem in that the controllability in particle size is deteriorated. In the case of the aggregation process using the crystalline resin, it is necessary to obtain emulsified particles that some kind of a dispersant or a hydrophilic functional group is contained in the resin, and it is not preferred since the crystallinity of the resin is deteriorated.
The invention has been made in view of the foregoing circumstances to solve the problems associated with the conventional toner, particularly a full color toner, for developing an electrostatic latent image, and is to provide:
(1) A toner for developing an electrostatic latent image and a developer for developing an electrostatic latent image that has a wide fixing temperature range and are excellent in fixing property at a low temperature;
(2) A toner for developing an electrostatic latent image and a developer for developing an electrostatic latent image that are excellent in charging property, particularly in environmental stability and time-lapse stability;
(3) A toner for developing an electrostatic latent image and a developer for developing an electrostatic latent image that can be easily produced, and are excellent in reproducibility of the particle shape and the particle size distribution and excellent in production stability;
(4) A toner for developing an electrostatic latent image and a developer for developing an electrostatic latent image that are excellent in production stability and storage stability of the binder resin particles;
(5) A process for producing the toner for developing an electrostatic latent image in a stable manner;
(6) A process for enabling stable formation of an image by using the toner for developing an electrostatic latent image; and
(7) An apparatus for enabling stable formation of an image by using the toner for developing an electrostatic latent image.
According to an aspect of the invention, the toner for developing an electrostatic latent image contains a crystalline resin having a melting point as a binder resin, and the toner further contains at least one compound which is selected from (a) an ester compound having an alkyl group having from 6 to 32 carbon atoms and (b) a resin having a contact angle with water that is smaller than that of the crystalline resin.
According to another aspect of the invention, the process for producing a toner for developing an electrostatic latent image contains the steps of: mixing by agitating a binder resin particle dispersion and an aggregated particle stabilizer dispersion to prepare an aggregated particle dispersion containing the binder resin particles; and heating the aggregated particle dispersion to a temperature higher than a melting point of a crystalline resin contained in the binder resin to form toner particles.
According to a further aspect of the invention, a process for forming an image contains the steps of: forming an electrostatic latent image; developing the electrostatic latent image with a developer to form a toner image; transferring the toner image to a fixing substrate; and fixing the toner image to the fixing substrate. In the process, the toner for developing an electrostatic latent image described in the above aspect is used to form the toner image.